Recently, HSDPA (High Speed Downlink Packet Access) or the like capable of increasing a packet transmission speed in a downlink (from base station to terminal) direction of about 384 kbps to 2 Mbps up to 14.4 Mbps at maximum is used for communication of a mobile station such as a mobile terminal.
The HSDPA automatically selects suitable modulation scheme and coding scheme according to a radio wave condition. For example, if the radio wave condition is bad, QPSK (Quadrature Phase Shift Keying) with high stability but low bit-rate is used, while if the radio wave condition is good, modulation is performed by using higher-bit-rate 16QAM (16 Quadrature Amplitude Modulation).
Furthermore, the HSDPA changes an error correction method in such a manner that when the radio wave condition is bad, a low-code-rate coding scheme with a high error correction capability is used, and that when the radio wave condition is good, then a high-code-rate coding scheme with a low error correction capability is used. Moreover, as a retransmission control method, the HSDPA adopts an HARQ (Hybrid Automatic Repeat Request) scheme to suppress the number of retransmissions upon detection of an error.
Here, a general operation of downlink HARQ will be explained by exemplifying LTE (Long Term Evolution) as a next-generation mobile phone system. First, a base station transmits a control channel (PDCCH) including downlink scheduling information (DL Scheduling Information) and a downlink data channel (PDSCH) including downlink transmission data.
At this time, a terminal receives the downlink data channel according to the scheduling information, and notifies the base station of ACK (success of reception) being a transmission acknowledgement signal ACK (acknowledgement) or NACK (failure of reception) using an uplink channel based on a result of Cyclic Redundancy Check (CRC). The base station, when receiving the ACK, stops HARQ, and retransmits data (transmits error correction bits different from the initially transmitted data) when receiving the NACK. Then, the terminal combines the initially received data with the retransmitted error correction bits, so that an error correction capability can be improved. If there is no error, then the data is transmitted at one time, while if there is an error, then the error can be corrected with a less number of data retransmissions. Therefore, the terminal can increase an average throughput.
However, if reception of PDCCH fails, the terminal cannot recognize transmission of PDSCH and cannot perform a reception process of PDSCH, and therefore, transmission of ACK/NACK is not performed. More specifically, when receiving ACK/NACK from the terminal, the base station needs to perform three-value determination including DTX being a state in which the terminal fails to receive PDCCH including “DL Scheduling Information” and does not transmit ACK/NACK, in addition to two-value determination of ACK/NACK.
In the uplink data communication, the base station transmits uplink-channel transmission information (UL Grant) to the terminal using PDCCH, and the terminal transmits uplink data channel (PUSCH) including uplink transmission data to the base station based on the information. But when a timing of transmitting ACK/NACK and a timing of transmitting the uplink data channel (PUSCH) coincide with each other, the terminal needs to simultaneously transmit ACK/NACK and PUSCH to the base station. However, the uplink data signal PUSCH in LTE uplink is transmitted on a single carrier in order to realize a low PAPR (Peak to Average Power Ratio). Therefore, the simultaneous transmission of ACK/NACK and PUSCH in a multiplexing manner on a frequency axis causes an increase in PAPR. Thus, the terminal needs to temporally multiplex ACK/NACK and uplink data on a data signal inside the PUSCH and transmit the data.
From the above, for example, in LTE, ACK/NACK is overwritten in the data signal of PUSCH and is transmitted. More specifically, as represented in FIG. 12, the terminal creates uplink data according to the uplink-channel transmission information (UL Grant), and when there is ACK/NACK information to be transmitted, part of the data is overwritten with ACK/NACK symbol and transmitted. At this time, if the number of symbols for ACK/NACK is increased, then a code rate of ACK/NACK bits becomes smaller, so that receiving characteristic is improved. Therefore, in LTE, the number of symbols used for ACK/NACK transmission is changed according to MCS (Modulation and Coding Scheme) specified by “UL Grant”. More specifically, to keep constant the reception quality of ACK/NACK, if the MCS is large, the reception quality is good, and therefore the number of symbols for ACK/NACK is decreased, and if the MCS is small, the reception quality is bad, and therefore the number of symbols for ACK/NACK is increased.
Furthermore, there is also used a technology for always ensuring symbols for ACK/NACK transmission. More specifically, as represented in FIG. 13, a time domain for transmitting data is always limited regardless of whether ACK/NACK is transmitted. In this case also, the number of symbols is usually changed by MCS.
Such conventional technologies are disclosed in for example “http://www.3gpp.org/ftp/Specs/html-info/36212.htm”, 3GPP TS36.212 V8.4.0 Chapter 5.2.2, and “http://www.3gpp1.com/ftp/tsg_ran/WG1_RL1/TSGR1—50b/Docs/”, 3GPP R1-074331.
However, the conventional technology has a problem that there is a trade-off between data transmission efficiency and ACK/NACK determination accuracy, and that high ACK/NACK determination accuracy cannot be ensured without a decrease in efficiency.
More specifically, in the case of the conventional technology as represented in FIG. 12, a transmission side (e.g., terminal), when ACK/NACK is not transmitted because of failure of reception of PDCCH, enters data for PUSCH into a portion of which ACK/NACK is supposed to be transmitted, and transmits the data. Therefore, in a reception side (e.g., base station), because values of bits in ordinary data are usually randomly different from bits of ACK/NACK after being coded, it is possible to perform three-value determination of ACK/NACK/DTX using the randomness. However, if the number of symbols for which ACK/NACK are multiplexed is large enough, then satisfactory determination accuracy of the three-value determination can be obtained. Meanwhile, if the number of symbols is small, then there arises a problem that the determination accuracy is largely degraded even if an SNR (Signal to Noise Ratio) is large.
In the case of the conventional technology as represented in FIG. 13, the transmission side (e.g., terminal) always ensures symbols for ACK/NACK transmission, and thus can ensure satisfactory determination accuracy if the SNR is large even if the number of symbols is small. Meanwhile, if there are a large number of symbols in particular, then there arises a problem that the number of symbols for PUSCH is always largely reduced and this causes low use efficiency of uplink resources.